Many approaches for waste disposal are currently available. For example, sanitary landfills formed by filling a land area with successive layers of solid waste and layers of earth or soil are well known. Unfortunately, such landfills have the potential for producing large amounts of a hazardous, explosive gas (methane), which may migrate to buildings or structures several hundred feet from the landfill if not removed from the landfill. Further, when this gas escapes into the atmosphere, it is about twenty times more potent as a greenhouse gas as carbon dioxide, which can have significant negative environmental effects. The natural precipitation draining out of the landfill may carry toxic, polluted water to contaminate underground water supplies, surface streams, and wells. Due to the very slow stabilization of waste, a landfill may not be used for other purposes for long periods of time and, thus, particularly near metropolitan areas, represents a large waste of land.
Other approaches utilize anaerobic digestion for stabilization and conversion of organic wastes to methane and compost. Anaerobic digestion is a series of processes in which microorganisms degrade and convert organic material in the absence of oxygen to produce usable gas and substrate. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich digestate (compost) can be used as fertilizer.
The designs and strategies employed to enhance anaerobic biogasification of organic feedstocks has been thoroughly researched in the last two decades. Among design options, each has its own set of benefits and constraints and the selection process is usually dependent upon feedstock characterizations and/or personal preference. Designs usually depend on factors such as reactor solids concentration, mixing strategy, temperature and number of stages (Chynoweth and Pullammanappallil, 1996, Anaerobic digestion of municipal solid wastes. In Palmisano, A. C. and Barlaz, M. A. eds. Microbiology of Solid Waste. CRC Press, Inc. Boca Raton, Fla., p. 71-113; Gunaseelan 1997, Anaerobic digestion of biomass for methane production: a review. Biomass and Bioenergy 13, 83-114; Mata-Alvarez and Llabres, 2000, Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Biores. Technol. 74, 3-16).
Single-stage anaerobic digestion (also known as one-phase system) involves a single housing in which the organic substrate and the microorganisms are contained. These systems are limited because they require continuous handling of feedstock (especially when total solids (TS) >20%). Moreover, there is little control over the reactions occurring in the system. The biogas produced in one phase systems consists primarily of carbon dioxide in the early stages of digestion, the high carbon dioxide content being attributable to the slow growth of the methanogenic microorganisms. The growth and proliferation of methanogenic microorganisms are further limited in single-stage systems as a result of high concentrations of volatile fatty acids (VFAs) due to hydrolysis and fermentation of macromolecules by acidogenic microorganisms. Thus, the biological reactions of different species in a single stage reactor can be in direct competition with each other. When the acidogenic and methanogenic processes are not synchronized, the entire system can shut down or the maximal methane gas yield is only achieved after longer retention times (Sarada and Joseph, 1995, A comparative study of single and two stage processes for methane production from tomato processing waste. Processing Biochemistry 31(4), 337-340).
In order to reduce the inhibition of methanogenic microorganisms by the high concentration of VFAs produced during acidogenesis, the two-phase digester has been introduced. Two-phase systems permit much higher organic substrate loads and have been proven to run at lower retention times than single-stage systems. FIG. 1 shows the block-flow diagram of a two-phase system (Azbar and Speece, 2001, Two-phase, two-stage and single-stage anaerobic process comparison. Journal of Environmental Engineering, March, 240-248). In literature, a “two phase system” and “two stage system” tend to be used interchangeably. For the purposes of this application, the two-phase and two-stage will be treated as separate processes.
A two-phase system refers to the optimization of different digestion vessels to bring maximum control over the bacterial communities living within the digesters. In this process, fermentation (acidogenic bacteria) and methanogenesis (methanogenic bacteria) are performed in separate reactors and distinguished by using different retention times. Typically, hydrolysis, acetogenesis, and acidogenesis occur within the first reaction vessel. Methanogenesis occurs in the second vessel.
Solid organic material (biomass) is added to the first vessel so that the biomass can be broken down into smaller constituents by acidogenic bacteria. The digested organic material from the first vessel is then often heated to an optimal temperature for methanogenic breakdown and pumped into the second vessel. Most biogas from the anaerobic two-phase system is collected from the second vessel, although a small amount of biogas may be produced in the first vessel. Unfortunately, with existing two phase systems, there are higher operational and maintenance requirements than that of single phase systems. For example, time for digestion (also referred to as retention time) in two phase systems can be long. This is generally due to the need for retaining the organic material in the first vessel for a period of time prior to addition to the second vessel to allow for proper hydrolysis/acidogenesis. Further, because a large portion of biogas is produced in the second vessel, there is untapped potential in having the first vessel also produce useable biogas.
Further, some digester systems are batch systems (see, for example, process known as sequential batch anaerobic composting or SEBAC). With these systems, biomass is added to a reactor at the start of the process in a batch and is sealed for the duration of the digestion process. Unfortunately batch reactors often suffer from odor issues that can be a severe problem when the reactor is emptied. Moreover, depending on the fermenting material and temperature, gas production from a batch-feeding will begin after two to four weeks, gradually increase to a maximum output and then fall off after about three or four months. Thus, often two or more batch digesters must be used in combination so that at least one will always be producing gas. To address this issue, continuous digestion processes were developed. With continuous digestion, organic matter is constantly added, or added in stages, to the reactor. The end products are constantly or periodically removed, resulting in constant production of biogas.
A common operational problem associated with any anaerobic digestion system is “foaming ” Foaming is the trapping of fine bubbles of gas in the semi-liquid digestion contents of a reactor. Foam forms primarily when the carbon dioxide-to-methane ratio is higher than normal. This usually occurs during start-up operations, but it can occur whenever a fresh food supply suddenly contacts live organisms. Currently, the only effective solution to dealing with this problem is to establish a two phase digester system.
Thus a system and method for more efficient anaerobic digestion of organic materials, particularly plant and crop wastes, that addressed the foaming issue and enabled greater production of usable biogas from both reactors in a two phase system would represent a significant advance. Quite surprisingly the present invention provides such systems and methods.